In the context of the present disclosure:                an intruder aircraft should be understood to be an aircraft not forming part of the formation and risking coming into collision with at least one aircraft of the formation, if no avoidance maneuver is implemented; and        a formation flight is considered to comprise at least two aircraft, notably transport airplanes, namely a trailed aircraft, called lead aircraft, and one or more trailer aircraft. The trailer aircraft fly following the aircraft that they trail directly (namely the lead aircraft or another trailer aircraft) so as to maintain a constant spacing between them.        
In a preferred application, in particular in cruising flight, the aircraft fly one behind the other at the same flight level with the same heading and the same speed. Provision can also be made to apply to the trailer aircraft speed control commands which are such that they enable the trailer aircraft to have the same position, the same speed, and the same acceleration as the trailed aircraft had with given prior durations.
Such a formation flight offers advantages particularly in terms of fuel consumption. In effect, the formation is generally defined, particularly in terms of separation distance, such that a trailer aircraft is not disturbed by the wake turbulences created by the one or more aircraft, in particular the lead aircraft, preceeding it in the formation.
Wake turbulence or vortex should be understood to mean an aerodynamic turbulence (or vortex) which is formed downstream of an aircraft. The wake turbulence can be generated by the gases ejected by the engines. It can also be created at the end of a wing of an airplane, because of the pressure difference between lower and upper surface and the downward deflection of the flow which results therefrom. A wake turbulence can also appear on parts forming angles with the plane of the wings, such as, for example, control surfaces when they are lowered.
Now, during such a formation flight, an aircraft external to the formation may happen to converge towards at least one of the aircraft of the formation so as to create a risk of collision requiring the implementation of a separation maneuver.
Airliners are equipped with anti-collision systems of TCAS (“Traffic Collision Avoidance System”) type which make it possible to ensure the safety of the air traffic by preventing the risks of collision in flight. Thus, when two aircraft converge towards one another, their anti-collision systems compute an estimation of the collision time and emit an alert informing the crews of each aircraft of a possible future collision: such an alert is generally called “traffic advice” or “TA alert”. If necessary, the anti-collision systems also emit, for the attention of the crew, a vertical avoidance maneuver command in order to exit from the potential collision situation: such an avoidance maneuver command is generally called “resolution advice” or “RA alert”. The TA and RA alerts are embodied by voice messages and by the display of information in the cockpits.
Such an avoidance maneuver generally consists of applying a vertical separation between the aircraft concerned generally of 700 feet for airliners.
Now, such a maneuver generating a change of altitude of at least one of the aircraft of the formation may, depending on the relative positions of the aircraft of the formation and of the intruder aircraft, require a trailer aircraft to cross the wake turbulence (or vortex) created downstream of one of the aircraft (trailer aircraft or lead aircraft) preceding it.
Such a situation can, for example, occur in a formation with two aircraft, when the trailer aircraft is situated at a lower altitude than that of the lead aircraft and the intruder aircraft is arriving from below which then, to perform the avoidance maneuver, requires the trailer aircraft to climb and cross the wake turbulence, if the altitude difference necessary for the avoidance is greater than the altitude difference of the aircraft in the formation. This potential crossing of a wake turbulence prevents, for instability reasons, the keeping of the formation. In such a situation, the avoidance maneuver will break the formation, such that the aircraft can no longer benefit from the abovementioned advantages of the formation flight.
Such standard management of an avoidance maneuver for a formation flight upon a risk of collision is not therefore satisfactory.